The present invention relates to printer devices, and particularly, although not exclusively, to a method and apparatus for servicing printing nozzles in page wide array ink jet devices.
As is well known in the art, conventional inkjet printers generally employ one or more inkjet cartridges, often called xe2x80x9cpensxe2x80x9d, which shoot drops of ink onto a page or sheet of print media. For instance, two earlier thermal ink ejection mechanisms are shown in U.S. Pat. Nos. 5,278,584 and 4,683,481, both assigned to the present assignee, Hewlett-Packard Company. The pens are usually mounted on a carriage, which is arranged to scan across a scan axis relative to a sheet of print media as the pens print a series of individual drops of ink on the print media. The series of drops collectively form a band or xe2x80x9cswathxe2x80x9d of an image, such as a picture, chart or text. Between scans, the print medium is advanced relative to the scan axis. In this manner, an image may be incrementally printed.
A continuing goal of inkjet printing technology is to increase the speed (i.e. reduce the time) with which an image may be printed. Various factors limit the speed with which an image may be printed. Amongst these factors is the time that the printhead carriage requires to scan across the print media. This time is especially important in unidirectional print modes, which are usually used to achieve high print quality. In unidirectional print modes, ink is printed only whilst the carriage is moving in one direction along the scan axis. Thus, for every printed swath, a non-printing return movement of the carriage along the scan axis is required.
One known method of avoiding this limitation is to use a page wide array (PWA) of printheads. In PWA printers, an array of printheads extending across the width of the page is used. Thus, ink may be ejected across the entire printable width of the print media, without moving the printheads across the width of the page. Generally, the print medium is then fed in a direction perpendicular to the array of printheads while the array of printheads is maintained stationary. In this manner, such scanning times may be eliminated.
In order to maintain the quality of the printed output of the printer device, it is important that each instruction to the print head to produce an ink drop from a given nozzle does indeed produce such an ink drop. Thus, it is important to verify that each nozzle is functioning correctly.
Optical drop detection systems are known, such as are used in the Hewlett-Packard DesignJet 1050 and Hewlett-Packard DesignJet 5000. In such systems, the printheads are periodically moved to a position above an optical drop detector, which situated outside of the print zone. One by one, the nozzles of a given printhead are controlled to eject ink drops through the light beam of the optical detector. If no ink drop is detected, the nozzle concerned may be assumed to be malfunctioning and appropriate maintenance routines may be implemented.
Such drop detection systems, although effective, suffers from the drawback that the drop detection procedure is time intensive and that while the procedure is being carried out, it is not possible to print. In the case of PWA printers such a procedure is not generally practicable. This is because the large numbers of nozzles in the page wide printhead arrays will require even longer to test than the printheads of conventional scanning printers, such as the Hewlett-Packard DesignJet 1050. Furthermore, however, since PWA inkjet printers aim to provide increased throughput, relative to conventional scanning inkjet printers, they are less tolerant to printing downtime.
It would therefore be desirable to provide an improved system and method for drop detection.
According to the present invention there is provided a printer apparatus comprising one or more printing elements arranged to print on print media in a print zone and a transport path arranged to transport printed sheets from the print zone to an output position, the apparatus further comprising a scanner arranged to scan a printed sheet, and diverting means arranged to selectively divert a printed sheet to the scanner, the scanner being located such that the apparatus may continue to print and transport sheets to the output position during the scanning process.
By printing marks on print media and transporting the printed media offline (i.e. away from the media path along which print jobs are fed once printed) to a scanner arranged to scan the marks, a drop detection process may be achieved by comparing data relating to the scanned marks with data relating to marks printed by a correctly functioning system. However, various advantages may be realised.
Firstly, there is no requirement to move print head or other printing elements of the printer away from the position in which they are arranged to print, in order to undertake a drop detection procedure. Thus, the time taken to implement the procedure may be comparatively small compared to prior art systems, which in turn ensures that the impact on the throughput of the printer is reduced.
Secondly, by transferring the sheet upon which a drop detection pattern has been printed off-line, the scanning process may be carried out at a different rate or speed compared to the on-line process or processes. By xe2x80x9con-line processxe2x80x9d, it is meant any serial, or sequential, process that may be carried out in a printer workflow. This may include various treatments, for example lamination, being trimmed or edged etc. However, it may simply include the activity of feeding the printed output away from the print zone (i.e. to an output position) at approximately the same speed as the printing process is being carried out; thus, allowing the printing process to continue efficiently and possibly allowing the printed output to dry. This means that the scanning process, and thus the drop detection process, may be carried out to a higher standard than would be the case if it were carried out on-line. Alternatively, it means that the drop detection process may be carried out to a satisfactory standard using equipment and/or techniques that are less expensive or demanding than would otherwise be required.
Thus, in a preferred embodiment, where the throughput of the printer is high, for example using a page wide array of inkjet nozzles, a conventional optical CCD scanner may be used to carry out the drop detection process without adversely impacting on the throughput of the printer; i.e. the printer may continue to print and feed the printed output in a normal manner at a given speed while the test pattern is being scanned at a lower speed.
Advantageously, conventional CCD scanning elements, such as are used conventional in photocopying devices are available with a relatively wide field of view. This allows an entire test pattern to be scanned in a single scanning pass, thereby reducing the time and complexity of scanning the sheet. Furthermore, by using a CCD type scanner, drop detection results may be more accurate than have historically been obtained in printers belonging to the inkjet field, for example. CCD type scanners are capable of detecting small drops of ink on print media by virtue of their higher resolution capabilities, for example. Additionally, a large gamut of colours may be detected through the implementation of a red, green, blue (RGB) charge coupled device (CCD) contained in the optical scanners.
Preferably, the printer is adapted to use different types of media for drop detection routines and for printing print jobs printed by the user. By using less expensive print media for servicing operations, the cost of the drop detection routines in terms of the consumables used may be further reduced. In one preferred embodiment, this is implemented using two or more media specific input trays.
In a particular embodiment of the invention, by allowing sheets of print media upon which test patterns are printed to be diverted back to the print zone along a further media feed path, one or more further test patterns may be printed on the same sheet; for example on the reverse side. By reusing such sheets, the cost of such servicing routines, may be reduced in terms of the cost in consumables.
The present invention also extends to the corresponding method. Furthermore, the present invention also extends to a computer program arranged to implement the present invention in conjunction with suitable hardware.